1. See Varner (1999,
pp. 51-54) for a discussion of the phylogenetic distribution of the
mechanisms underlying pain. However, caution is recommended with
respect to his table on p. 53 which suggests that the presence of
specialized nociceptive neurons is limited to vertebrates. The
existence of nociceptors seems to be well established in marine
molluscs. (See E. T. Walters [1992] “Possible clues about the
evolution of hyperalgesia from mechanisms of nociceptive sensitization
in aplysia,” in Hyperalgesia and Allodynia, W. D.
Willis, Jr. (ed.), New York: Raven Press.)

2. While the locution
“feel pain” suggests conscious experience to most readers,
it does not do so for all. For instance, Carruthers (1998b) maintains
that although animals do in fact feel pain, they do not feel pain
consciously — or, as Carruthers somewhat paradoxically puts
it, he denies that the feeling feels like anything. Many philosophers
would, however, charge that the concept of an unconscious pain is
conceptually incoherent, pain being essentially conscious.

3.
Because of the importance of animal sentience to ethics, some of the
most explicit statements of the evolution-reinforced similarity
argument can be found in the animal rights and animal welfare
literature (e.g., Singer 1975/1990; Regan 1983; Andrews 1996; Varner
1999).

4. It is interesting to note that
many species, especially among the cetaceans and migratory birds, have
been determined to sleep with each hemisphere of their brains
independently. This adaptation enables marine mammals to avoid
drowning and birds to keep flying. We do not know of any systematic
study of how perceptual and cognitive capacities are affected when
half the brain is in a sleep state.

5. This problem goes under
many names; Allen & Bekoff (1997) refer to this as “the
other species of mind problem” and Prinz (2005) calls it
“The Who Problem”.

6. The
evidence for this is somewhat ambiguous, however. For example,
Güven Güzeldere (1995) reports private correspondence from
David Armstrong who wrote:

… following Locke and Kant, I think the introspective
awareness is perception-like. For instance, it is very like
proprioception, and seems not to involve any linguistic capacity.
Perhaps chimpanzees and even dogs have such consciousness. [G.
Güzeldere (1995, fn. 22), “Is consciousness the perception of what
passes in one's own mind?”, in Conscious Experience, Thomas
Metzinger (ed.). Paderborn: Schöningh/Imprint Academic, 1995, fn.
22.]

And Lycan, commenting on Carruthers (1998a) in his contribution to the
Psyche symposium on animal consciousness writes:

Even if I am right about human beings, of course it does
not follow that other animals exhibit a comparable degree of
computational complexity. Perhaps some do and some do not; perhaps few
if any do. I would continue to maintain that an animal has
phenomenal-consciousness in the strong sense if and only if that animal
has HOEs [Higher Order Experiences]. So I would at least provisionally
conclude that if many animals (including very young human children)
lack the computational complexity needed for HOEs, those same many
animals lack phenomenal-consciousness in the strong sense. Carruthers
would not disagree with that.

These are hardly ringing endorsements by higher-order experience
theorists for animal consciousness.

7. Hauser,
Chomsky, and Fitch (2002) argue that comparative language studies have
been hampered by a failure to distinguish three separate dimensions
for comparing the human faculty of language in the broad sense (FLB)
to other forms of animal communication. They label these three
dimensions “sensory-motor”,
“conceptual-intentional”, and “FLN-recursion”
(FLN= faculty of language, narrow). Their article provides a useful
review of evidence for considerable evolutionary homology and
homoplasy in the first two dimensions. In a follow-up study to
investigate the third dimension, Hauser & Fitch (2004) found that
while their tamarin monkeys could discriminate strings conforming to a
simple finite state grammar which allows arbitrary repetitions of a
simple pattern (e.g., ABn), but they could not master a
phrase structure grammar (such as AnBn). However,
Gentner et al. (2006) found that starlings could discriminate
AnBn patterns (although these authors may have
over-interpreted this as mastering a recursive rule, since phrase
structure grammars do not require full recursion). See also Allen and
Saidel (1998) and Trout (2001) for discussions by philosophers of
alternative possible evolutionary relationships between human language
and animal communication, and Anderson (2004) for a linguist's
perspective.

8. This argument is very
reminiscent of Davidson's (1975) argument against animal thought on
the grounds that one must have the concept of thought to be able to
think. We are not aware that Davidson was ever tempted to turn this into
an argument against animal sentience.

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